This invention relates to phased array antennas for generation of multiple beams by the use of beamformers, and particularly to such phased array antennas in which each element of the array is associated with a transmit-receive module.
Radar systems often use monopulse techniques to derive angle tracking information from a single echo pulse. This is accomplished by generating two, or more usually three, antenna beams, so that the simultaneously received echoes from the multiple beams can be compared A phased-array antenna which generates three simultaneous beams in order to support monopulse operation requires a separate beamformer for each of the three beams.
High performance phased array monopulse antennas in current use may have several thousand elemental antennas or antenna elements in the array. A beamformer for use with such an antenna array must therefore include several thousand inputs, and may require different amplitude weighting for signals applied to each input, which amplitude weighting may change depending upon operating conditions. The amplitude weighting is provided in order to control the sidelobe performance of the beam or beams.
Those skilled in the antenna arts know that transmission and reception by antennas are reciprocal operations, and that the operation of antennas is similar in both operating modes. However, descriptions of antenna operation are usually couched in terms of either transmission or reception, with the other mode of operation being understood.
Several thousand inputs of each beamformer of a large phased array antenna must be phase controlled in order to control the beam direction of the antenna array. The interconnections within the beamformer, which lie between the antenna and the common output port of the beamformer, must be carefully phase controlled so as not to introduce errors. When three such beamformers are used, one to produce a sum (.SIGMA.), azimuth difference (AZ .DELTA.) and elevation difference (EL .DELTA.) beams, the errors due to the beamformers must track closely enough so that desired performance is achieved under all operating conditions.
U.S Pat. No. 4,866,449 issued Sept. 12, 1989 in the name of Gaffney describes a scheme for testing a monopulse system and for processing detected signals in such a manner as to correct for time delay differences. The magnitude of the corrections required to be made by such a scheme would be reduced if the errors attributable to the beamformers were reduced.
FIG. 1 is a simplified block diagram of a prior art monopulse radar system. In FIG. 1, radar system 10 includes an antenna array 12 including individual antennas or antenna elements 14.sup.1, 14.sup.2, 14.sup.3, ...14.sup.N-2, 14.sup.N-1, and 124.sup.N arrayed in a column designated 16.sup.1. Other columns 16.sup.2, 16.sup.3....16.sup.N are illustrated in a general manner as being located behind column 16.sup.1, so as to form a two-dimensional rectangular array of antenna elements.
Each antenna element 14.sup.1, 14.sup.2...14.sup.N of columns 16.sup.1, 16.sup.2...16.sup.N of antenna array 12 is associated with a phase shifter 18. For example, elemental antenna 14.sup.1 of column 16.sup.1 is associated with a phase shifter 18.sup.1. Similarly, each of the elemental antennas 14.sup.22, 14.sup.3...14.sup.N of column 16.sup.1 are associated with a phase shifter 18.sup.2, 18.sup.3...18.sup.N. As also illustrated in FIG. 1, phase shifter 18.sup.1 has an output transmission line (cable) 20.sup.1 which, together with output cable 20.sup.N of phase shifter 18.sup.N of column 16.sup.1, is connected to a sum-and-difference hybrid circuit 22.sup.1. Each of cables 20.sup.1 and 20.sup.N is connected to a separate input port (input) of hybrid circuit 22.sup.1. It will be noted that phase shifters 18.sup.1 and 18.sup.N are associated with elemental antennas 14.sup.1 and 14.sup.N, the first and last (top and bottom) antenna elements of column 16.sup.1. Similarly, the output of phase shifter 18.sup.2 is coupled by way of a cable 20.sup.2 to a second sum-and-difference hybrid splitter 22.sup.2, together with the output from phase shifter 18.sup.N-1, coupled by way of a cable 20.sup.N-1. Phase shifter 18.sup.2 is associated with antenna element 14.sup.2, the second antenna element, and phase shifter 18.sup.N-1 is associated with penultimate antenna element 14.sup.N-2. A third sum-and-difference hybrid combining arrangement 22.sup.3 receives inputs from the third antenna element 14.sup.3 and its phase shifter 18.sup.3 by way of cable 20.sup.3, and from antepenultimate antenna element 14.sup.N-2 and its phase shifter 18.sup.N-2 by way of cable 20.sup.N-2,, respectively. It can be seen that the outputs of the antenna elements of column 16.sup. 1 and their phase shifters are taken in pairs symmetrically disposed above and below the center of column 16.sup.1, and the antenna outputs are combined in an array of sum-and-difference hybrids. The combination or array of sum-and-difference hybrids 22 associated with column 16.sup.1 is designated 24.sup.1.
Each of the other columns, such as column 16.sup.2, 16.sup.3...16.sup.N, includes (not illustrated) its own column array of antenna elements 14 and phase shifters 18, each of which is associated with an antenna 14. Each of the other columns is also associated with an array 24 (not illustrated) of sum-and-difference hybrids 22. Only antenna array column 16.sup.N is illustrated in FIG. 1 as being connected by cables 20 to its associated sum-and-difference hybrid array 24.sup.N.
In the arrangement of FIG. 1, the sum output produced at the upper output of hybrid 22.sup.1 of hybrid array 24.sup.1, is coupled by way of a cable 26.sup.1 to an input of a sum combiner or beamformer 30.sup.1. Similarly, the upper or sum (.SIGMA.) outputs of sum-and-difference hybrids 22.sup.2 and 22.sup.3, and all the other hybrids (not illustrated) of hybrid array 24.sup.1, are coupled by a cable 26 to sum combiner 30.sup.1, which combines the sum signals, and which couples the combined sum signals to a single output cable 34.sup.1. Similarly, the difference (.DELTA.) output ports of sum-and-difference hybrids 22.sup.1, 22.sup.2, 22.sup.3,...22.sup.n/2 of hybrid array 24.sup.1 of FIG. 1 are each connected by way of a transmission line 28 to separate inputs of a difference combiner or beamformer 32.sup.1. Thus, the .DELTA.(lower) output port of hybrid 22.sup.1 is connected by way of a cable 28.sup.1 to a first input of .DELTA.combiner 32.sup.1, the a output port of hybrid 22.sup.2 is coupled by way of a cable 28.sup.2 to a second input of .DELTA.combiner 32.sup.1, and the .DELTA.output port of hybrid 22.sup.3 is coupled by cable 28.sup.3 to a third input of .DELTA.combiner 32.sup.1. All the other hybrids (not illustrated) of hybrid array 24.sup.1 have their .DELTA.output ports coupled to a .DELTA.combiner 32.sup.1 in a similar manner. Combiner 32' combines the ' signals and couples their sum to an output cable 36'.
Each of the other hybrid arrays 24.sup.2...24.sup.M (only 24.sup.M illustrated) of FIG. 1 are connected to an associated pair of sum and difference combiners or beamformers in the same manner. The M.sup.th hybrid array, namely 24.sup.M, is illustrated in FIG. 1, together with some of its cables 20, and also with some connection 26 to last column .SIGMA. combiner 30.sup.M.
As so far described, all the columns 16.sup.1 through 16.sup.M ultimately produce a sum signal from a column sum combiner 30 on a cable 34, and a difference signal from a column .DELTA. combiner 32 on a cable 36. Thus, there are M cables 34, and M cables 36, one for each column 16.
Elemental phase shifters 18 can be adjusted so that the input signals to column .SIGMA. combiners 30 add in-phase for a desired antenna beam pointing direction. Difference signals to column .DELTA. combiner 32 will add in-phase only if cable pairs 26.sup.N and 28.sup.N are phase matched for all N, provided that the .SIGMA. and .DELTA. combiners for each column have identical topologies.
First cable 34.sup.1 and last cable 34.sup.M from sum combiners 30.sup.1 and 30.sup.M, respectively, are coupled to individual inputs of a sum-and-difference hybrid designated 38.sup.1. The outputs from the second (30.sup.2) and penultimate (30.sup.M-1) combiners (not illustrated) are coupled over cables 34.sup.2 and 34.sup.N-1 to separate input ports of a second sum-and-difference hybrid 38.sup.2. Similarly the third (30.sup.3) and antepenultimate (30.sup.M-2) sum combiners 30 (not illustrated) have their outputs coupled by way of cables 34.sup.3 and 34.sup.M-2, respectively, to a sum-and-difference hybrid 38.sup.3. Other sum-and-difference hybrids (not illustrated) together with hybrids 38.sup.1, 38.sup.2, and 38.sup.3, form an array 40.sup.M of sum-and-difference hybrids. Each hybrid of array 40.sup.M receives inputs from a pair of column sum combiners 30 associated with a pair of columns 16, the columns of which are symmetrically disposed to the left and right of the center of array 12.
The sum outputs of the hybrids of hybrid array 40.sup.M of FIG. 1 are each separately coupled by way of a cable 44 to a separate input of an azimuth sum combiner 48. For example, hybrid 38.sup.1 has its .SIGMA. output connected by way of a cable 44.sup.1 to an input of azimuth combiner 48, hybrid 38.sup.2 has its .SIGMA. output connected by a cable 44.sup.2 to another input of azimuth combiner 48, and hybrid 38.sup.3 has its .SIGMA. output connected by way of a cable 44.sup.3 to a third input of azimuth sum combiner 48. Azimuth sum combiner combines the .SIGMA. signals and produces the combined .SIGMA. signal on a cable 50 for application to a processing and display unit illustrated as 70.
The .DELTA. outputs of each of sum-and-difference hybrids 38 of hybrid array 40 of FIG. 1 are each separately coupled by way of a cable 46 to separate inputs of an azimuth .DELTA. combiner 52. For example, the .DELTA. output of hybrid 381 is connected by way of a cable 46.sup.1 to an input of azimuth .DELTA. combiner 52, the .DELTA. output of hybrid 38.sup.2 is connected to a second input of azimuth .DELTA.combiner 52 by way of a cable 46.sup.2, and the .DELTA. output of hybrid 38.sup.3 is connected by way of a cable 46.sup.3 to yet another input of combiner 52. Combiner 52 combines the .DELTA. signals and applies the combined signals over a cable 54 to processing and display unit 70 of radar unit 10.
Another array 41 of sum-and-difference hybrids, each of which is designated as 42 in FIG. 1, is coupled to the array of M column .DELTA. combiners 32 (only combiner 32.sup.1 is illustrated), in much the same fashion that array 40 of hybrids 38 is coupled to an array of M sum combiners 30. For example, sum-and-difference hybrid 42.sup.1 receives inputs by way of cables 36.sup.1 and 36.sup.M from first and last column .DELTA. combiners 32.sup.1 and 32.sup.M (not illustrated). Sum-and-difference hybrid 42.sup.2 is connected by way of cable 36.sup.2 and 36.sup.M-1 to the second and penultimate column .DELTA. combiner 32 (not illustrated), and hybrid 42.sup.3 has its inputs connected by way of cables 36.sup.3 and 36.sup.M-2 to the third and antepenultimate column .DELTA. combiners 32. Other hybrids 42 of array 41 are connected to other pairs of combiners symmetrically disposed to the left and right about the center of array 12.
The sum outputs of each of sum-and-difference hybrids 42 of array 41 of FIG. 1 are coupled by way of separate cables 56 to separate inputs of an elevation .DELTA. combiner 62. For example, hybrid 42.sup.1 has its sum output connected by way of a cable 56.sup.1 to a first input of combiner 62, and the sum outputs of hybrids 42.sup.2 and 42.sup.3 are connected by separate cables 56.sup.2 and 56.sup.3, respectively, to other inputs of elevation .DELTA. combiner 62. Elevation .DELTA. combiner 62 combines the column .DELTA. signals to produce an elevation .DELTA. signal on a cable 64 for application to processing and display unit 70.
The difference (.DELTA.) outputs of sum-and-difference hybrids 42 of hybrid array 41 of FIG. 1 are not used and are terminated. For example, the .DELTA. output of hybrid 42.sup.1 is coupled by way of cable 58.sup.1 to a termination 60.sup.1, and the .DELTA. outputs of hybrids 42.sup.2 and 42.sup.3 are coupled by cables 58.sup.2 and 58.sup.3 to terminations 60.sup.2 and 60.sup.3, respectively.
A transmitter 72 associated with radar system 10 of FIG. 1 is coupled to processing and display unit 70 for timing the signals, for providing appropriate demodulation reference signals, and for other purposes. Also, a transmitter signal is applied to cable 50 of azimuth sum combiner 48, as suggested by dotted lines 74 within processing and display unit 70. The transmitter signals are coupled through azimuth combiner 48 and back through the arrays of hybrids and combiners, which in the context of transmission may act as splitters, to ultimately produce signals at antenna elements 14, which signals are phased in a manner appropriate for directing radiation in a particular direction.
The complexity of the beamforming arrangement of FIG. 1 is apparent. Additional complexity arises because of the amplitude weighting of the signals relative to each other in each column 16, and from column to column, in order to achieve the appropriate sidelobe levels for both elevation and azimuth beams. Even if phase shifters 18 are set correctly, assuming equal phase signals arriving at the phase shifters, cumulative phase errors through the combiners and hybrid arrays may adversely affect the performance. In this regard, it should be noted that the actual physical lengths of interconnecting cables such as 20.sup.1, 20.sup.2...20.sup.M must be nearly equal for wide bandwidth signals, and some cables such as 26.sup.N and 28.sup.N must have the same electrical length as well, even though the distances over which the signals must be carried may be less than the physical lengths This in turn tends to create a problem relating to excess cable lengths associated with the shorter paths, which excess cable lengths must be stored out of the way. An improved beamforming arrangement is desired.